This invention relates to an inner sealing material for closing the mouth of a container and, more particularly, to an inner sealing material of a type that is to become effective by high-frequency sealing.
The mouths of containers of powdered or granular materials such as instant coffee, powder milk and flavorings are fitted with an inner sealing material which is fitted into the cap of the container for protecting the contents from deterioration in quality due to various factors such as contact with moisture and oxidation. The inner sealing material is a combination of a reseal layer that is based on an elastic sheet such as cardboard and a membrane layer (seal layer) that is based on a metal foil such as aluminum foil. When the container is untapped, the reseal layer is separated from the membrane layer and remains in the cap whereas the membrane layer is left on the mouth of the container. When the cap is replaced on the mouth of the container, the reseal layer is superposed on the membrane layer (which may be removed either partly or entirely) so that it combines with the cap to prevent the entrance of moisture, oxygen, etc., into the container.
To bond the membrane layer onto the mouth of the container, emulsion based adhesives have heretofore been used, but this has involved several problems when the user attempts to open the container by peeling the membrane layer from its mouth. To name a few, "picking", or the destruction of the glassine paper used in the membrane, occurs, and difficulty is encountered in peeling the membrane layer neatly. With a view to solving these problems, a high-frequency sealing method has been developed as a technique that enables the mouth of a container to be sealed with a thin film that can be bonded to the mouth in a fast and easy way and which yet can be peeled neatly as required at a later time (see, for example, Japanese Patent Publication (kokoku) Nos. Sho 61-9181 and 63-28019, and Hei 3-2754).
This high-frequency sealing method causes the metal layer in the inner sealant to generate heat when it is placed in a magnetic field and the thus generated heat is utilized to fuse the thermal bonding resin layer (glass seal layer) at the bottom of the membrane layer, thereby providing an effective seal. However, if this method is applied to the conventional inner sealants, serious problems have occurred in association with the ability to insure effective separation between the reseal and membrane layers, as exemplified by the loss in the ability of the release layer (which forms the topmost part of the membrane layer) to insure good release on account of heat generation, and inconsistency in peel strength that is caused by thermal dissolution of the layer to be bonded which forms the bottommost part of the reseal layer.